Water vapor is water in gaseous form, which passes through water separators and coalescing filters just as easily as compressed air. The ability of air to hold water vapor is dependent upon its pressure and its temperature. The higher the temperature, the more water vapor that can be held by the air; the higher the pressure, the greater amount of water vapor is squeezed out.
As large volumes of air are drawn into the compressor and compressed, the temperature of the air increases significantly. This allows the heated air to easily retain the water vapor in the atmospheric air. Unlike water in a liquid or aerosol form (which is removed from compressed air using a coalescing filter), water vapor — water in a gaseous form — can only be removed using a dryer.
Compressed air purification equipment must deliver uncompromising performance and reliability while providing the right balance of air quality with the lowest cost of operation. Many manufacturers offer products for the filtration and purification of contaminated compressed air, which are often selected only for their initial purchase cost, with little or no regard for the air quality they provide, the cost of operation throughout their life or even their environmental impact.
When purchasing purification equipment, delivered air quality, the overall cost of ownership and the equipment's environmental impact must always be considered.
Desiccant dryers remove water vapor by passing air over a regenerative adsorbent material, known as desiccant, which strips the moisture from the air. All desiccant air dryers remove water vapor using this method; however, the adsorbent must be periodically regenerated to ensure a continuous supply of dry air, and a number of different regeneration methods are available.
Pressure dewpoint is the term used to describe the temperature at which condensation will occur. The water removal efficiency of a dryer is expressed as a pressure dewpoint (written as a temperature). Desiccant dryers are highly efficient and typically provide pressure dewpoints of -40°F (-40°C) or -100°F (-70°C). This means that in order for water vapor to condense into a liquid, the air temperature would need to drop below -40°F (-40°C) or -100°F (-70°C) respectively. The temperature of the compressed air after it has passed through a desiccant dryer is not the same as its dewpoint.
Typically, a pressure dewpoint of -40°F (-40°C) is used in most applications. Compressed air with a dewpoint below -14.8°F (-26°C) will not only prevent corrosion, it will also inhibit the growth of micro-organisms within the compressed air system.
Desiccant air dryers (also known as regenerative air dryers) can be purchased in a tower tower dryer or modular dryer construction. Types include: heatless dryer, heated dryer, blower purge dryer and a high pressure air dryer. Point of use dryer and inline air dryer varieties bring clean, dry air just where you need it at the point of use application.
In general purpose applications, refrigerated air dryers are often used in place of desiccant air dryers. Refrigerated air dryers remove water vapor by cooling the compressed air and collecting the condensed liquid. For this reason, they are limited to to positive pressure dewpoints to prevent freezing of the condensed liquid, providing pressure dewpoints of +37.4°F (+3°C), +44.6°F (+7°C) or +50°F (+10°C).
This type of dryer will remove free water from the system and slow corrosion. However, they will not inhibit bacterial growth and are not suitable for installations where piping is installed in ambient temperatures below the dryer dewpoint (i.e. systems with external air receivers and piping). Types include but are not limited to: thermal mass dryer and high temperature refrigeration.